1. Field of the Invention
The present invention generally relates to the field of implantable heart stimulation devices, such as pacemakers, implantable cardioverter-defibrillators (ICD), and similar cardiac stimulation devices. More specifically, the present invention relates to an implantable cardiac stimulator arranged for dual chamber pacing in a first mode with atrial ventricular synchronized stimulation, comprising mode switch means for switching from the first mode to a second mode without atrial ventricular synchronized stimulation.
2. Description of the Prior Art
In a dual chamber pacing system, e.g. a DDD-R system, atrial events are tracked for ventricular stimulation synchronization. The purpose is to mimic the behavior of a healthy heart, where the natural ventricular contractions are synchronized with atrial contractions, and to thereby obtain a physiologically optimized ventricular stimulation. However, the pacing system should not track the atrial activity during atrial tachyarrhythmias, since an atrial synchronized ventricular stimulation would not meet the physiological needs of the patient. The pacing mode should in this case be changed to a non-atrial tracking mode, such as VVI-R, where physiologically adequate stimulation therapy would be provided, albeit not physiologically optimized.
Cardiac stimulators are known in the art which are provided with a mode switching feature adapted to stabilize the ventricular heart rate during atrial fibrillation or other atrial condition where atrial tracking is not appropriate. In response to a detection of atrial fibrillation, such a stimulator therefore switches into a non-atrial synchronized ventricular rate stabilization pacing mode. The stimulator remains in this mode of operation as long as the atrial fibrillation prevails.
The detection of atrial tachyarrhythmia, such as atrial tachycardia (AT), atrial fibrillation (AF), atrial flutter or supraventricular tachycardia (SVT), is generally obtained from an atrial electrogram (IEGM). In order to avoid unfounded triggering of mode switch, the detection algorithms are generally set such that the atrial tachyarrhythmia must persist for a period of time before it is reliably detected and a mode switch is triggered. This is also generally the case for the opposite situation, i.e. when a stabilization of atrial behavior is determined and a mode switch back to atrial tracking is triggered. Thus, the atrial tachyarrhythmia must be absent for a certain period of time before a return to atrial tracking mode is triggered.
Furthermore, in order to avoid over-sensing, that is to erroneously indicate atrial tachyarrhythmia for triggering a mode switch, the detection thresholds are often set high in order to increase specificity. However, this often results in under-sensing, which is failure to detect actual atrial tachyarrhythmia that could, and for the benefit of the patient often should, result in a mode switch. One particular instance where the risk of under-sensing is apparent is the occurrence of atrial fibrillation, AF. This is mainly due to the fact that the amplitude of the IEGM is very low during AF. Thus, the AF condition, as well as other tachyarrhythmia conditions, may go undetected.
An effect of having a delay between the onset of atrial tachyarrhythmia and the triggering of a switch to a non-atrial tracking mode is that fast atrial rhythms may be conduced to the ventricles and result in an unphysiologically fast ventricular rhythm. Obviously, this may be unpleasant for the patient. Furthermore, the delay in mode switch back to atrial tracking may result in a period of undesired physiologically inadequate pacing therapy.
If the detection and triggering algorithms were to be set such that the delay is significantly shortened, the above problems could be reduced. However, the chance of unwarranted mode switch would increase, which would not be beneficial in terms of optimizing pacing therapy with a stable and physiologically correct atrial behavior. Consequently, there exists is a trade-off between speed and specificity of mode switch triggering.
Thus, there is a need for improved mode switch triggering algorithms which reduces mode switch delays without sacrificing specificity in atrial tachyarrhythmia determination.